BM-40 (BM40), also known as osteonectin or SPARC (secreted protein
acidic and rich in cysteines), consists of a short stretch of acidic
amino acids, a region homologous to follistatin and an extracelluar
calcium (EC) binding module. At least four other extracellular
glycoproteins -- testican, tsc, QR1 and SC1 -- all of unknown function,
have homologous follistatin and EC modules. The latter half of the EC
module, residues 209-286, consists of a pair of EF-hands (_-helices D,
E, F & G of the EC module) as confirmed in the crystal structure .
The second EF-hand binds calcium in canonical coordination. The first
EF- hand binds calcium in the third (after RLC of molluscs and S100)
known variation on the canonical pattern of calcium binding. Since
BM40 is extracellular, calcium binding supposedly imparts stability,
not information, to it. The cleft between the two EF-hands is occupied
by the first _-helix of the EC module, 140-160, in an orientation very
similar to that of a target helix bound by calmodulin; this appears to
be another example of stabilization, not information transduction.